This document provides an overview of various non-destructive testing (NDT) methods, including visual inspection, liquid penetrant testing, magnetic particle inspection, ultrasonic testing, radiographic testing, eddy current testing, acoustic emission testing, and thermal imaging. It describes the basic principles, equipment, applications, advantages, and limitations of each method. The key NDT methods covered are visual inspection, radiographic testing, ultrasonic testing, and eddy current testing.
2. Visual-Optical Inspection
Borescope: (i) Rigid (ii) Flexibile
(a) Rigid and fixed – Series of achromatic relay lenses in an optical tube with
an incandescent lamp for illumination: Tube dia – 4-70 mm. Mag – 3-50X
(b) With rotating shaft having light guide bundle for orbital scan
3. Flexible fiberscope : Dia 1.4 – 13mm. Consists of light guide bundle (optic
fibers of 30 µm dia) and image guide bundle with interchangeable viewing
heads. May also consists of a CCD image sensor at the distal tip.
6. Cold lap
Incomplete fusion
Lack of penetration
RT Image Weld defects
Cold lap is a condition where the weld filler metal
does not properly fuse with the base metal or the
previous weld pass material (interpass cold lap). The
arc does not melt the base metal sufficiently and
causes the slightly molten puddle to flow into the base
material without bonding.
Incomplete penetration (IP) or lack of penetration
(LOP) occurs when the weld metal fails to penetrate
the joint. It is one of the most objectionable weld
discontinuities. The appearance on a radiograph is a
dark area with well-defined, straight edges that
follows the land or root face down the center of the
weldment.
Incomplete fusion is a condition where the weld filler
metal does not properly fuse with the base metal.
Appearance on radiograph: usually appears as a dark line
or lines oriented in the direction of the weld seam along
the weld preparation or joining area.
7. Porosity
Internal or root under
cut
Offset or mismatch
Internal or root undercut is an erosion of the base
metal next to the root of the weld. In the radiographic
image it appears as a dark irregular line offset from the
centerline of the weldment. Undercutting is not as
straight edged as LOP because it does not follow a
ground edge.
Offset or mismatch are terms associated with a
condition where two pieces being welded together are
not properly aligned. The radiographic image shows a
noticeable difference in density between the two pieces.
The difference in density is caused by the difference in
material thickness. The dark, straight line is caused by
the failure of the weld metal to fuse with the land area.
10. Digital Radiography
Flat panel detectors (FPD)
Phosphor plate, barium fluorohalide activated by europium. Latent image is
excited by laser, photomultiplier converts the light out put to an analog image
which is converted to a digital one by A/D converter.
11. Direct conversion of X-ray to electrical signal - a two-dimensional
array of thin film transistor (TFT) with an X-ray converter. Amorphous
Si (TFT) coated with a image-conversion layer (CsI)
The light from phosphor falls on photodiode which creates charges
proportional to light intensity. They get deposited on signal storage capacitors.
Row of TFTs (pixels) – Gate line – Stores the charge
Columns – Data or Drain line – Transfers the charge
TFTs in ‘Off’ mode during exposure. A positive bias after exposure opens the
gates (Transfers the charge to Data lines D1 ----- Dn) gate-by-gate.
15. 2-D and 3-D cross-sectional images of an object
Images are taken at different angles by rotating the sample as if the sample is sliced at
different planes and imaged. Tomography - Tomos – slice graphein – to write
When the density data of these slices are stretched and put together (reconstructed) they
create a 3D image of the object or the defects
16. Radiation safety
Exposure vaults
Exposure cabinets
Safety control -
Engineered control – Shielding, door interlocks, alarms, warning
lights. Rope and signal portable radiography
Administrative control –
Postings, procedure, dosimetry and training
24. Surface probe – ‘Pancake’ type OD or Encircling Coils ID or Bobbin Probes
25. Reference standard
Should be manufactured from the same material type, alloy, material thickness,
and chemical composition that will be found on the component to be inspected.
Sizes and tolerances of flaws introduced in the standards are usually regulated by
inspection specifications.
Common eddy current reference standards include:
Conductivity standards.
Flat plate discontinuity standards.
Flat plate metal thinning standards (step or tapered wedges).
Tube discontinuity standards.
Tube metal thinning standards.
Hole (with and without fastener) discontinuity standards.
Narrow notches produced with electron discharge machining (EDM) and saw cuts are
commonly used to represent cracks, and drilled holes are often used to simulate corrosion
pitting.
3 or 4 Al plates
fastened together
in a lap join
fashion, 2nd
or 3rd
layer contains EDM
notches or
naturally/artificially
induced cracks
26. Thermal Imaging
wavelength of thermal radiation extends from 0.1 microns to several hundred microns.
Heat transfer
Conduction – solid
Convection – liq, gas
Radiation – no medium
27. Infrared (IR) radiation has a wavelength that is longer than visible light - greater than 700
nanometers.
As the wavelength of the radiation shortens, it reaches the point where it is short enough to
enter the visible spectrum and can be detected with the human eye.
An infrared camera has the ability to detect and display infrared energy.
The basis for infrared imaging technology is that any object whose temperature is above 0 K
radiates infrared energy.
28. Emissivity
Measure of a surface’s ability to radiate infrared energy
ratio of thermal energy emitted by a surface to the energy emitted by a perfect
blackbody at the same temperature.
Equipment
Detector and display system
Detector – Thermal and Quantum
Thermal – Heat sensitive coating that melts at certain temp or change color with
temp. Thermoelectric or pyroelectric devices like thermocouple, thermistors or
bolometers
Thermopile
Bolometer
29. IR incoming photons of a certain wavelength absorbed by the sensor and creates
electron-hole pair which can be detected as electrical current. Thus radiation
energy is directly converted to a signal.
But signal output is very small and hence suffers from noise. Since noise is partly
proportional to temperature, detector is operated at cryogenic temp.
Photoconductive and photovoltaic
Photoconductive – generates electrons, holes or electron-hole pairs
Indium antimonide (InSb), quantum well infrared photodetector (QWIP, GaAs/AlGaAs
multilayer), mercury cadmium telluride (mercad, MCT), lead sulfide (PbS), and lead
selenide (PbSe).
Photovoltaic - require an internal potential barrier (p-n junction or Schottky
barriers) with a built-in electric field in order to separate photo-generated
electron-hole pairs.
indium antimonide (InSb), mercury cadmium telluride (MCT), platinum silicide (PtSi), and
silicon Schottky barriers
30. Imaging
Scanning a detector (or group of detectors) –
single element detector scanning along each line in the frame (serial scanning)- requires
very high scan speed
series of elements are commonly scanned as a block, along each line
The frame movement can be provided by frame scanning optics (using mirrors) or in the
case of line scan type imagers, by the movement of the imager itself
Focal plane array (FPA) - group of sensor elements organized into a rectangular grid.
The entire scene is focused on the array, each element cell then provides an output
dependent upon the infrared radiation falling upon it.
The advantage of FPAs is that no moving mechanical parts are needed and that the
detector sensitivity and speed can both be slower. The drawback is that the detector
array is more complicated to fabricate and manufacturing costs are higher.
31. A special lens focuses the infrared light emitted by all of the objects in view.
The focused light is scanned by FPA detectors. The array of detector lies in the focal
plane of the lens.
A detailed temperature pattern called a thermogram, is created based on the radiation
energy falling on each sensor. This information is obtained from several thousand points
in the field of view of the detector array.
The thermogram created by the detector elements is translated into electric impulses.
The impulses are sent to a signal-processing unit, a circuit board with a dedicated chip
that translates the information from the elements into data for the display.
The signal-processing unit sends the information to the display, where it appears as
various colors depending on the intensity of the infrared emission. The combination of all
the impulses from all of the elements creates the image.
32. Thermal imaging devices
Un-cooled - This is the most common type of thermal-imaging device. The
infrared-detector elements are contained in a unit that operates at room
temperature. natural or artificial pyroelectric materials, usually in the form of a thin
film, out of gallium nitride (GaN), (CsNO3), polyvinyl fluorides or micrbolometers.
This type of system is completely quiet, activates immediately and has the battery
built right in.
Cryogenically cooled - these systems have the elements sealed inside a
container that cools them to sub zero temperature. The advantage of such a
system is the incredible resolution and sensitivity that result from cooling the
elements. Cryogenically-cooled systems can detect a temperature difference as
small as 0.1 C from a distance of more than 1,000 ft (300 m).
33. Image interpretation
White areas indicate hot spots and black areas indicate cooler regions
Modern equipments are capable resolving the temperature in terms of
different colors
34. Application to NDT
Thermal image technique will detect defects or anomalies in the underlying material as changes
in the surface temperature. Owing to different thermal coefficients between defects and the
surrounding material, heat will flow in different way and with different rate, and the result will be
a different temperature distribution in correspondence of defects.
Passive and active method
Passive - No external heating. Heat is generated due mechanical load (stress). cracks tips or stress
concentration zones are areas where heat is accumulated and dissipated and they appear as hot
spots.
Active – part to be tested is heated externally to get radiation from the surface.
Single sided – sample is heated by an external heat source and then sensor is scanned to
record the radiation
Double sided – The heat source and the sensor are placed on opposite sides and operated
simultaneously
36. Brazed joint of dissimilar metals
Brazed joint of cooling tube to a heat sink
Thermal image X-ray image
37. Civil structures
Bright patches indicate missing and poorly positioned insulation in the walls of a new
industrial unit.
Survey of concrete roof slab in full sun. Dark (cool) patches indicate presence of moisture
trapped in upper insulation layers
38. Can be used for inspecting parts in service e.g. Mechanical and
Electrical Systems – Simply pointing an infrared camera at a component
and looking for areas of uneven heating and localized hot spots.
Electrical – loose connection, failed transformer
Mechanical – improper bushing and bearing lubrication,
over loaded motor or pumps, coupling misalignment
Electronics – temperature of the junction is a critical
factor for the life of semiconductor device
39. Corrosion damage:
material thinning of relatively thin structures e.g. aircraft fuselage
Surface being inspected needs to be heated – out put obtained from different
parts will give an idea about the damage. Heating is done xenon flash lamps.
Heat will be conducted away from the surface faster from thicker region. From the
temp profile a thickness map can be generated
Corrosion damage and disbonding in the inside
surface of an aircraft skin
40. Gas turbine engines liners
SiC-SiCf liner with EBC
Defect (delamination )
delamination/dedonding of the EBC layers after
13,937 hrs of engine operation
2 mm2
/s 17 mm2
/s
Correlation of cross-sectional photomicrographs
with one-sided thermal image showing locations
where EBC debonding and pre-spall occur
41. Flaw Detection
Sound material, a good weld, or a solid bond will see heat dissipate rapidly
through the material, whereas a defect will retain the heat for longer.
Vibrothermography image of a 7075 Al plate
Vibrothermography – exciting the sample by bursts of high energy, low frequency
acoustic waves. this generates frictional heat at the faces of a crack and hence the crack
can be detected by thermal imaging
42. Process monitoring
no water for cooling. There are no
stripes from improper headers.
New headers were installed to quench
slab, however, the header nozzles were
not properly installed causing hot and cold
stripes.
Steel slab
43. Product quality
On the tyre testbed: infrared
camera detects rolling resistance
and heat development of tyre
Audi cars
Monitoring heat distribution in
the catalytic converter
Thermal imaging of the engine testbed to find out temp
distribution and potential failure sites
